by John Walsby

15K Drawing of eels by J. Walsby

Most of our native freshwater fish - such as inanga, kokopu, koaro and mudfish - are quite small and little known because compared with introduced trout and salmon, they are tricky to catch, are not good game fish and offer too little for the table.

The juveniles of these fish, especially of inanga, were once very well known. As whitebait returning from the sea to spend their adult lives in freshwater, they were caught in millions ending up in fritters or as snacks on cocktail biscuits.

Today they are less common because there are far fewer adults. It is likely that this is because of changes to our waterways. Many of the wetlands around little creeks and side streams have been reclaimed or had the shady bush removed from their banks. Elsewhere waterways have been polluted with silt and chemicals. The silt is washed down from bare land where forests have been clear felled or where there is crop farming or residential and industrial development. It clogs the gills of small animals and also restricts the growth of microscopic plant plankton by clouding the water and cutting out daylight.

Eels are by far our largest native freshwater fish and seem more tolerant of murky waters than other native species. They are therefore often associated with mud holes and dank backwater seepages but like other freshwater fish they like and need oxygenated water that is free of pollutants.

They are extremely long bodied fish with just one pair of lateral fins and have narrow dorsal and ventral fins that join as a continuous tape around the tip of the tail. Of the 16 different species found in the world, only two, the longfinned and shortfinned eels, are native to New Zealand. The longfinned is only found in this country but shortfinned eels also live in Australia and other islands of the South Pacific.

The long finned eel is distinguished by the dorsal fin extending much farther forward than the ventral one and it has a larger mouth. The dorsal fin in the short finned eel starts just in front of the ventral.

Eels live from 12 to more than 50 years depending on their species and mature adults are sometimes very large. Shortfinned eels grow up to a metre long and weigh about 3á5kg, though most are a little smaller when they return to the sea to breed after 12 to 15 years living in freshwater.

The longfinned eel is one of the world's largest. Today after decades of fairly intensive fishing the very large specimens that were once caught are rare. An eel measuring 1á25m long and weighing 10kg would be considered large now but in the past, 15 kg, 1á5m long eels were not exceptional considering that a few eels 2m long and weighing up to 50kg have been caught. Eels this large are probably well over 50 years old.

An eels autumn migration back to the sea to breed is its last journey. It stops feeding, becomes darker and undergoes a number of body changes. Its bulbous head becomes flattened and pointed, its eyes almost double in size and its pectoral fins become larger.

They breed in deep water far away from land, often towards the tropics. There each female may lay a couple of million eggs from which small leaf-shaped larval fish hatch. Swimming and drifting in ocean currents it probably takes these larvae over a year to return to New Zealand coastal waters where they change quickly to long slender fish but remain almost transparent.

These little finger length juveniles, called glass eels, move into our estuaries in the spring. There over about a fortnight, they undergo internal bodily changes and develop into pigmented young eels, called elvers. The elvers remain in the estuaries for a while, then, moving mostly at night, they steadily migrate upstream. After a year or more they eventually arrive at the upper reaches of streams and rivers and at some inland lakes, negotiating rapids, natural waterfalls and even the spillways of some hydroelectric dams on the way.

Young eels feed mainly on insect larvae, small snails worms and midges. Only when they are quite large, many years later, do they start to feed on larger prey such as koura and small fish. Very large eels are also known to take ducklings and are very sensitive to the smell of carrion upstream, a fact that eel fishermen exploit when baiting their traps with blood and offal.

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